TFT have heretofore been being used as switching elements in active matrix-type liquid crystal devices (hereinafter referred to as AMLCD). At present, devices with TFT circuits that comprise active layers of amorphous silicon films have a high market share. In particular, inverse stagger structures capable of being produced in simple processes are much employed for constructing TFT.
With recent developments in high-quality AMLCD, however, TFT are being required to have much better operating characteristics (especially for high operating speed). In such situations, amorphous silicon TFT are often unsatisfactory as their operating speed is not high, and high-quality devices comprising amorphous silicon films are difficult to produce.
Accordingly, polysilicon TFT have become much highlighted in place of amorphous silicon TFT, and TFT comprising polysilicon films as the active layers are being actively studied and developed in the art. At present, some polysilicon TFT devices are on the market.
Many reports have already been disclosed, relating to inverse stagger-type TFT structures comprising active layers of polysilicon films. For example, referred to is a report of "Fabrication of Low-Temperature Bottom-Gate Poly-Si TFTs on Large-Area Substrate by Linear-Beam Excimer Laser Crystallization and Ion Doping Method: H. Hayashi, et al., IEDM 95, pp. 829-832, 1995".
In that report, they illustrated one typical example (FIG. 4) of inverse stagger structures comprising polysilicon films. However, inverse stagger structures of that type (that is, so-called channel-stop-type ones) have various problems.
First, in those structures, the active layers having an overall thickness of 50 nm or so are extremely thin. Therefore, in those, impact ionization at the junction of the channel-forming region and the drain region is occurred, whereby the structures are significantly deteriorated due to hot carrier implantation. For these reasons, a large LDD region (light doped drain region) must be formed in those structures.
In this connection, the most critical problem is how to control the LDD region. The LDD region requires extremely delicate control of the impurity concentration therein and the length of itself. In particular, the length control of the region is problematic. At present, the length of the LDD region is defined by mask patterning. In fine TFT, however, any minor patterning error in masking the LDD region will produce significant differences in TFT characteristics.
Another serious problem is that the sheet resistivity in the LDD region significantly varies depending on the variation in the thickness of the active layers. Moreover, the variation in the taper angle of the gate electrodes often causes the variation in the function of the LDD region.
In addition, the LDD region requires patterning, which directly complicates the production process while lowering the throughput. It is presumed that the production of the inverse stagger structure described in the report noted above requires at least 6 masks (up to the step of forming the source/drain electrodes).
As mentioned above, the channel-stop-type inverse stagger structure indispensably requires the transverse in-plane LDD region to be formed at the both sides of the channel-forming region, in which, however, a reproducible LDD region is extremely difficult to form.